This invention concerns a novel method and apparatus for determining fuel octane numbers. Many methods and devices for determining fuel octane numbers are known. However, none of them is entirely satisfactory because each method or device either requires too much time or is not accurate enough. Engine drift, which is inherent in all test engines, is an important cause of these problems. Engine drift occurs when a knock test engine that is run on a particular fuel with constant running and measuring conditions generates knock intensity signals which go through intensity changes. Such intensity changes occur according to deposit build-up and flaking, changes in operating temperature, changes in voltage supplied to the knock measuring equipment and mechanical shifts in the engine. Because of the changes occurring during the normal manual octane test procedure, the operator has many opportunities to exert bias by selecting the moment at which the reading is taken.